Conventionally, inspection of wafers and liquid crystal display substrates (hereinafter, collectively "substrates") is performed by an inspector who directly holds a substrate in relation to a point light source. The inspector then attempts to visually detect defects such as scratches, foreign matter, unevenness in the coating of resist and abnormalities in the transfer pattern of the substrate surface. Among such defects, some are observed while others are not observed, depending on the direction of the rays incident the substrate, and the direction of the line of sight of the inspector. Consequently, the inspection of defects is performed by rotating and inclining the substrate relative to the light source. However, foreign matter like sweat and dirt may adhere to the substrate being inspected because it is directly handled by the inspector. Thus, inspection apparatus have been proposed wherein the substrate to be inspected is suction-clamped to a holder capable of rotating and inclining the substrate under observation. This allows the inspector to observe the substrate without direct contact.
Inspection apparatus have been proposed wherein the shape of the light source for irradiating the substrate surface is planar or linear, making the defects easier to see. However, this type of sensory inspection ultimately depends on the vision of the inspector. Thus, the inspection criteria vary with the inspector's skill, level of fatigue and other subjective factors. This makes it difficult to consistently perform substrate inspection based on a fixed (i.e., objective) standard.
To solve the abovementioned problems, inspection apparatus have been proposed that aim to stabilize the inspection standard by automating the inspection of the substrate surface using image processing.
Briefly, in image processing apparatus for substrate inspection, a nearly parallel illumination irradiates the substrate surface from a predetermined direction. Specularly reflected light, diffracted light from the surface pattern, and scattered light from foreign matter, scratches, etc., on the substrate surface are condensed by a concave mirror. An imaging lens, as a light receiving optical system, forms an image of the substrate which is photoelectrically detected by an image detector, such as a CCD. The image detector then outputs an image signal to an image processing system, which detects unevenness in the coating on the substrate, abnormalities in the transfer pattern arising from defocusing when transferring a pattern (e.g., a mask pattern) onto the substrate, and scratches, as well as the presence of foreign matter on the substrate surface. This is accomplished by comparing the image signal from the image detector to information related to a previously stored normal, (i.e., reference) substrate surface image.
When starting up a manufacturing line for manufacturing semiconductor devices or liquid crystal displays, it is difficult to properly transfer the patterns onto the substrates. It is also difficult to form normal patterns over the entire substrate surface because of instabilities in the manufacturing process. Consequently, there is no choice but to detect defects of such substrates automatically by image processing. To perform feature extraction of defects by image processing, algorithms that recognize periodic structures as non-defects are often employed.
Preferably, the detection optical system in the inspection apparatus includes a concave mirror for condensing light from the substrate surface, and a light receiving optical system (e.g., an imaging lens) for guiding the condensed light to the image detector. To prevent the substrate from being in shadow, an off-axis construction is typically adopted so that the central axis (i.e., the optical axis of the detection optical system) through which the center of the light beam from the substrate passes, and the optical axis of the concave mirror form a predetermined angle. In this way, the imaging lens does not overlap the substrate.
However, if this type of off-axis construction is adopted, the image of the substrate surface formed on the light receiving surface of the image detector is distorted. Thus, if image processing is performed as is, such distortion may cause a false detection.
In addition, the detection optical system does not necessarily capture only the light beams proceeding in a direction perpendicular to the substrate surface (i.e., the substrate surface normal) when it receives specularly reflected light and diffracted light from the surface. Thus, the substrate surface image formed on the image detector becomes a tilted image. This compresses the image in the tilted direction. Consequently, if the tilted image is image processed after photoelectric detection by the image detector, false detection occurs.
There is also the problem that the compression ratio of the tilted image is not fixed. When the detection optical system receives diffracted light from the substrate surface, the tilt angle changes due to the pitch of the pattern formed on the substrate.